1. Field of the Invention
The present invention relates generally to apparatus and methods for producing low pressure plasmas. More particularly, the present invention relates to the production of a highly uniform planar plasma which can be used for treating semiconductor wafers in low pressure processing equipment.
Plasma generation is useful in a variety of semiconductor fabrication processes, including etching, resist stripping, passivation, deposition, and the like. Generally, plasmas may be produced from a low-pressure process gas by inducing an electron flow which ionizes individual gas molecules through the transfer of kinetic energy through individual electron-gas molecule collisions. Most commonly, the electrons are accelerated in an electric field, typically a radiofrequency electric field produced between a pair of opposed electrodes which are oriented parallel to the wafer.
The use of an electric field normal to the wafer to accelerate the electrons, however, does not provide efficient conversion of the kinetic energy to ions, especially at low frequencies and pressures below about 0.1 Torr. Under such conditions, a large portion of the electron energy is dissipated through electron collisions with the walls of the processing chamber or with the semiconductor wafer itself. The direct collision of electrons with the semiconductor wafer is not only energetically wasteful, but can also cause wafer heating which is highly disadvantageous.
Several methods have been proposed to increase the efficiency of plasma generation for use in semiconductor processing equipment. For example, microwave resonance chambers use ultra high frequencies (e.g., 2.45 GHz) which shorten the electron oscillation path and increase the likelihood of transferring the electron energy to the process gas molecules rather than the walls of the process vessel or the semiconductor wafer. Electron cyclotron resonance (ECR), in contrast, uses a controlled magnetic field to induce a circular electron flow within the process gas. While achieving relatively high energy conversion efficiencies, both these methods generate a highly non-uniform plasma which must be made uniform prior to exposure to the semiconductor wafer. Usually, a certain degree of uniformity can be achieved by flowing the plasma some distance prior to exposure to the wafer or wafers. The need to provide the additional flow path, however, allows some ion recombination which reduces the effectiveness of the plasma. Each system also suffers from pressure operating range limitations. Microwave resonance chambers are generally effective for process gas pressures from about 1 to 760 Torr, while ECR is effective from 0.0001 to 0.1 Torr. Moreover, the cost and design complexity of both systems are increased by the need to provide the extra flow distance, and the magnetic field required by the ECR system is difficult to control.
Other approaches for enhancing the efficiency of plasma generation in semiconductor processing equipment include magnetically-enhanced plasma systems (such as magnetically-enhanced reactive ion etching) and inductively-coupled electron acceleration, commonly called inductively-coupled plasma. Magnetically-enhanced plasma systems produce a constant magnetic field parallel to the wafer surface and a high frequency electrical field perpendicular to the wafer surface. The combined forces cause the electron to follow a cycloidal path, increasing the distance traveled relative to the straight path which would be induced by the electric field alone. This approach can provide good ion generation efficiency, but the large uniform magnetic field required for semiconductor processing is very difficult to maintain. Also, operation of the magnetically-enhanced systems is generally limited to a pressure range from about 0.01 to 0.1 Torr.
Inductively-coupled plasma processes also cause the electrons to follow an extended path. The term "inductively coupled plasma" is used for two different techniques, both using alternating current to transformer couple energy to a gas. The first uses a ferrite magnetic core to enhance transformer coupling between a primary winding and a secondary turn consisting of a closed path through the gas. This technique normally uses low frequencies, below 550 Khz. The second technique uses a solenoid coil surrounding a cylindrical gas to be ionized. This technique can use either low frequencies or frequencies in the range of 13.56 Mhz. Neither of these techniques provides a uniform plasma adjacent and parallel to a wafer surface.
For these reasons, it would be desirable to provide apparatus and methods for generating highly uniform plasmas within semiconductor processing equipment, including etching equipment, deposition equipment, resist stripping, and the like. The apparatus should be capable of generating a high flux plasma over a very broad pressure range, and the plasma so produced should have little or no directed ion energy. Optionally, the apparatus should be capable of imparting directed energy to the plasma ions, with the control of directed energy being separate from the control of plasma flux. It would be particularly desirable if the apparatus were of a relatively simple design, were easy to operate and control, and required minimum capital expense. Similarly, the methods should be straightforward and easy to implement and should provide a high quality product in a short time with minimum expense.
2. Description of the Background Art
Skidmore (1989), Semiconductor International June 1989, pp 74-79, is a review article describing electron cyclotron resonance (ECR) and magnetically-enhanced reactive ion etch (MERIE) systems. U.S. Pat. No. 4,368,092, describes a plasma generating system employing a helical inductive resonator for producing the plasma external to an etching chamber. The plasma is non-uniform and passes through a tube before utilization. U.S. Pat. No. 4,421,898, describes an inductively-coupled plasma generating apparatus, where a transformer having a magnetic core induces electron circulation in an insulating tube carrying a process gas. Gas ionization is non-uniform, and exposure to the wafer occurs downstream. U.S. Pat. No. 4,626,312, describes a conventional parallel plate plasma etcher where the wafer is situated on a lower electrode and a plasma is generated by applying radiofrequency energy across the lower electrode and a parallel upper electrode. U.S. Pat. Nos. 4,668,338 and 4,668,365, describe magnetically-enhanced plasma processes for reactive ion etching and chemical vapor deposition, respectively.